What is it?
Having two different eye colours refers to the term heterochromia iridum - a difference in eye colouration. Heterochtomia is a rusult of an excess or lack of pigments (melanin). There are two kinds of heterochromia: 1. Complete Heterochromia - one iris is a different colour then the other. 2. Partial Heterochromia - only part of one iris is a different colour then the rest.
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Complete Heterochromia
How Can This Happen?
Someone can have two different eye colours based on genestics, difficulty with pigment transport, puberty, trauma in the whomb or shortly after birth, and a medical syndrome known as Waardenburg syndrome. Also as you age, your eye colour can change. About 10-15% of the Causcasion population eyes will change as they age. As you get older and you notice your eyes are changing colour, it's recommended that you go visit your eye doctor. In some cases, the change in eye colour can lead into a disease.

How Eye Colour Relates to Optics or Light
Having two different eye colours relates to optics because it is part of the eye, and gives it colour.

By: Kimberly
References:
http://en.wikipedia.org/wiki/Heterochromia_iridum
http://www.allaboutvision.com/conditions/eye-color.htm
http://www.seniorwomen.com/news/index.php/why-do-eyes-change-color